Make ServerXMLSerializationTest DRYer

Move server's description info to setUp() to get rid of duplicate data.
Make user's and admin's attributes static class variables and
extract function which checks obligatory admin/user attributes.
Extract function which checks amount and values of fields in XMLized result.

Blueprint: nova-tests-code-duplication
Change-Id: Iffaec9555476cb8006130ec97f455d7810815970
Merge "Make ServerXMLSerializationTest DRYer"